We studied at necropsy atherosclerotic plaque composition in the 4 major (right, left main, left anterior descending, and left circumflex) epicardial coronary arteries in 15 patients who died of consequences of an acute myocardial infarction (AMI) and in 12 patients with sudden coronary death (SCD) with AMI. Within the AMI group and within the SCD group, there were no differences in plaque composition among any of the four major epicardial coronary arteries. Within both groups, plaque morphology varied as a function of cross-sectional-area narrowing of the segments. In both groups, the amount of dense relatively acellular fibrous tissue, calcified tissue, and pultaceous debris increased in a linear fashion with increasing degrees of cross-sectional-area narrowing of the segments, and the amount of cellular fibrous tissue decreased linearly. In the AMI group, the percentage of plaque consisting of pultaceous debris and of cellular fibrous tissue separated significantly narrowed (>75% cross-sectional area) segments from less narrowed (<75%) segments. A comparison of the AMI group to the SCD group showed significant differences.